1. Field of the Invention
This invention relates to a strain gauge sensor using the piezoresistive effect. It also concerns the process for manufacturing such a sensor.
2. Description of the Invention
A strain gauge is an element of a mechanical magnitude sensor which is deformed under the action of the mechanical phenomenon whose magnitude is to be measured, and this deformation causes a variation of its electrical resistance. This variation in the electrical resistance is used to deduce the value of the mechanical strain exerted. Semiconducting gauges based on the piezoresistive effect. have replaced metallic gauges due to their significantly better sensitivity.
At the present time, micro-machined silicon sensors are more and more frequently used due to their low cost and their high performances. The most developed of various types of sensors, since they are the best known, are undoubtedly those that use detection based on single crystal silicon strain gauges. They can be classified in two categories:
a first category includes sensors whose strain gauges are isolated from the substrate by an inverse pn junction and are made by diffusion of implantation of ions in the semiconducting substrate, PA1 a second category includes sensors whose strain gauge is isolated from the semiconducting substrate on which it is made, by a dielectric layer. The substrate used may be an SOI (silicon on isolator) substrate of the SIMOX, SDB or ZMR type, the dielectric layer being made of silica. PA1 etching of a first substrate of a semiconducting material with a given doping, along a crystallographic plane determined to improve its etching and to make up the said structure, PA1 manufacture of a strain gauge made of a semiconducting material with a freely chosen type of doping along a crystallographic plane determined to improve its piezoresistivity coefficient, PA1 operation in which the strain gauge is bonded onto the structure to obtain the said sensor.
U.S. Pat. Nos. 456,901, 4,739,298 and U.S. Pat. No. 4,510,671 describe embodiments of sensors in this second category. Document U.S. Pat. No. 4,739,298 describes a pressure sensor whose membrane and support frame are made of silicon. The membrane is covered by a thick layer of glass that is used for anodic gluing. The sensor is made by a set of plates. The strain gauge is made of highly p doped silicon, this choice being related to the use of a selective chemical etching step.
The advantage of sensors in the first category is their low cost, and the advantage of sensors in the second category is that they have high performance and can be used at a higher temperature. In each case, the crystallographic orientation of the strain gauge is necessarily identical to that of the substrate.
The authors of this invention concluded that these sensors, although very efficient, are not made in an optimum manner. The piezoresistance coefficient varies as a function of the orientation of the substrate plane, depending on the direction of this plane and on the type of doping of the semiconducting material. Furthermore, anisotropic chemical machining of the substrate to make the mechanical structure of the sensor also depends on the crystallographic orientation of the sensor. Therefore, there is a close interdependence between the support element and the sensor detection element. For a given crystallographic orientation of the substrate, it is possible to obtain a particular geometry for the machined mechanical structure and a particular direction for the sensitive axis of the strain gauge on this structure, consequently limiting optimization of the final component.
A reference on the behavior and the dependence of the piezoresistivity of silicon on its crystalline orientation is given in the article written by Y. KANDA entitled "A Graphical Representation of the Piezoresistance Coefficients in Silicon" published in the IEEE Transactions on Electron Devices, Vol ED-29, No. 1, January 1982. Anisotropic chemical etching of silicon is described in the article "Anisotropic Etching of Silicon" by K. E. BEAN published in IEEE Transactions on Electron Devices, Vol. ED-25, No. 10, October 1978.